The Date Modified in the items list is the modification date for the attachment title, not the file. You can see the modification date of the file itself in the right-hand pane after clicking on the attachment item.
Zotero will automatically upload files you've changed locally. You can just look at the file in the web library if you're using Zotero Storage to confirm that.
Upgrade Storage
Zotero will automatically upload files you've changed locally. You can just look at the file in the web library if you're using Zotero Storage to confirm that.